[linux-dvb] Re: Which CRC-code is used in TS of DVB?

[Rene Bartsch <ml@bartschnet.de>]

Am Die, 2003-05-27 um 02.06 schrieb Ralph Metzler:
> Gregor Lawatscheck writes:
>  > At 23:26 26/05/2003, you wrote:
>  > >The decoder does not get the TS.
>  > 
>  > AFAIK there is no CRC in the TS but in the PES header? I haven't checked 
>  > this, so I could be wrong.
> 
> No CRC in either.
> 

What is that? Quote from Holger Waechtler:

-------------------- snip ------------------------------

Hi,

As subject says, which CRC-code is used in TS of DVB?

I think I've read CRC32 somewhere - is that correct?
it's CRC32, the decoder model is described in ITU-T H.222.0, Annex A.

Holger

--------------------- snap ------------------------------

>  
>  > Nevertheless: 
>  > http://www.linuxtv.org/mailinglists/linux-dvb/2002/11-2002/msg00176.html is 
>  > interesting, from Oct Nov 2002 where Holger speaks about CRC32 checking 
>  > being broken in the av7110 firmware. Is this still the case and would that 
> 
> It is not broken, just not implemented.

There have been mails on the List from Holger Waechtler that it has been
reenabled - this was in October 2002. There also where mails on the
kernel-ML about impementing a generic CRC32 class into DVB-kernel.
> 
>  > 
>  > Because as Holger writes:
>  >  >It's implemented. Set the DMX_CHECK_CRC flag in the 
>  > dmx_sct_filter_params >struct if you want only receive sections that passed 
>  > the CRC32 check.
>  > 
>  > Now that sounds like it'll drop sections that didn't pass CRC32, doesn't 
>  > it? I'm not aware CRC32 corrects anything. I personally just fear CRC32 
> 
> Correct.
> 
OK, what passes the CRC-check, then?
> 
> 
>  > errors, for example in archives etc. To me it's a checksum: According to 
>  > http://www.cyberdyne-software.com/crc32.html CRC = "cyclic redundancy 
>  > check(sum)". What do you do when the checksum fails? "Invert[ing] the wrong 
>  > bits" is certainly beyond me.

The application's name is "CRC 32"!

I don't know the CRC32, and I'm currently looking for specs.

See

http://www.informatik.uni-leipzig.de/rnvs/lehre/rn1/DLL_AR02.pdf

for description of cyclic redundancy CODE.

> 
> Confused with FECs.

So the question: Is CRC32 in DVB-TS a cyclic redundancy checksum or
code? If checksum the DVB-developers would be idiots - especially as
data-packets are broadcasted by carussels where a client can't demand to
resend the packet - so it has to be valid.

The CRC32 is specified in Annex B of the MPEG-2 Systems ISO/IEC 13818-1
[1]. Can anyone check if it is checksum or code?

> 
> 
>  
>  > I actually thought the CRC32 was good exactly for the latter reason - error 
>  > detection - passing it to the decoder with the notice: "hey, by the way, 
>  > this data is broken, handle with care". I'd like to believe otherwise. It'd 
>  > be a great benefit to everyone if CRC32 from a budget card could be used to 
>  > correct bitstream errors.  I thought FEC (forward error correction) was for 
>  > error correction of the bitstream - and surely that's done even for the 
>  > budget cards... 
> 
> Yup, it is part of the demodulator hardware. 

With the full-featured cards only or also with low-budgets?

Rene



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